Extend ns_config:update so that it can do soft-deletes.

Change-Id: I095b4cb6c7217906998463ad11ea86fd92108ad6
Reviewed-on: http://review.couchbase.org/35287
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Soft-delete all per-node keys from config when shunning a node.

Change-Id: I358ed41f413810371e77cb40730174afe3998b20
Reviewed-on: http://review.couchbase.org/35288
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>